The ability of a swab cup to pick up light loads as well as heavy loads depends upon the cup's ability to deform in accordance with the operating load conditions of a particular well. For light load operation, the swab cup must deform with less force or fluid weight acting on the cup than is present during heavy load operation. In light load operation, the swab cup must deform under the light weight fluid load so the outer periphery of the swab cup contacts the interior wall of a well's tubing in order that the swab cup can pick up and transport well fluid through the well tubing. In heavy load operation, the interior portion of the swab cup is subjected to a substantially greater load than is necessary to expand the swab cup to sealing contact against the interior of the well tubing. In other words, when operating in a heavy load condition, the swab cup is basically subjected to an internal fluid pressure load in excess of that necessary to seal against the well tubing. The loading condition of a swab cup is dependent upon the height of fluid above the swab cup while it is being withdrawn from a well in addition to the velocity at which the swab cup is being withdrawn.
In prior art swab cup constructions, the internal reinforcing members have strongly supported locations around some portions of the swab cup body and weaker supported locations at other portions of the swab cup body. One such weaker location is immediately above the base portion of the internal support structure. As a result of this, some prior art swab cups will exhibit a tendency to perforate or blow out at a location immediately above this base portion when they are subjected to heavy fluid loading conditions.